Palaeontological and geochemical characteristics of Silurian black shale formations from the Central Brittany Domain of the Armorican Massif (northwest France)

Abstract

Within the Armorican Massif, as well as throughout the major part of the Gondwanan northern margin, Llandovery to Ludlow deposits are mainly represented by black shales with subordinate sandy beds. In Brittany, these euxinic Silurian facies lie more or less conformably on Late Ordovician deposits. However, based on graptolite evidence, a stratigraphical break corresponding to part or whole of the Llandovery is noticed. The Silurian black shales of Brittany are devoid of benthic fauna, while pelagic or epipelagic elements are very common. These euxinic lithofacies are characterized by peculiar geochemical compositions which are discussed in terms of their stratigraphical and geographical distribution. Significant enrichment of Sr, Y, Rb, Cr and Ni, higher concentrations of V (up to 5600 ppm) and greater volatile contents are particularly apparent in contrast with other terrigenous deposits from Brittany. A model for the sedimentary environment of the Silurian black shales of the Armorican Massif is proposed: the transgression induces the individualisation of basins partially separated from the open ocean in which the water column is stratified; then the stratification disappears progressively during Ludlow times.